Page 301 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
296 Geotechnical Engineering

compactive effort is required for soil to reach the specification density at the
optimum moisture content. This is reduced to about 50 percent at 2 percent above
optimum. However, compacting at above the OMC narrows the target range and
increases the possibility for overcompaction.

13.7.5 Relaxing Density Requirements
For applications where the compacted soil is not heavily relied on for strength and
where some settlement is permissible, such as for landscape grading that does not
involve steep slopes or high retaining walls, the density requirement may be
reduced to 90 percent of the maximum. In the example in Fig. 13.13, 90 percent
density could be achieved with about 40 percent effort at the OMC, or compared
with 65 percent for 95 percent density. The same criterion applied to the soil in
Fig. 13.12 would result in about a 3 percent reduction in volume upon saturation,
so saturating the lower 1 m (3 ft) of a fill placed by this criterion would generate a
settlement of 30 mm or a little over an inch.

13.7.6 Modified Proctor Density
The trend toward a higher density and lower optimum moisture content conti-
nues with modified Proctor compaction, which moves the density curve higher
and farther to the left (Fig. 13.14). The energy for laboratory compaction is
3
3
56,00 ft-lb/ft (2700 kN-m/m ) for the modified test compared with 12,400 ft-lb/ft 3
3
(600 kN-m/m ) for the standard test, an increase of over 450 percent. The
modified density requirement therefore is energy-intensive and is used only in
special circumstances.

Figure 13.14
Modified Proctor
density is
achieved with a
higher compactive
effort and a lower
moisture content,
and results in a
denser, stronger,
and less
compressible soil.
It is used in
heavy, duty
applications such
as airport landing
strips and
runways.

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